1. Technical Field
The present invention relates to a magnetic resonance imaging (MRI) system for medical use, and in particular, to a magnetic resonance imaging system capable of imaging a plurality of selective regions, such as multi-slice regions, while an object to be imaged is continuously moved in a certain direction.
2. Related Art
In general, magnetic resonance imaging (MRI) can be defined as the imaging technique that magnetically excites nuclear spins of an object placed in a static magnetic field with a radio-frequency signal at a Larmor frequency and reconstructs an image from MR signals generated due to the excitation.
Recently, in the field of the magnetic resonance imaging, there has been proposed a technique for imaging an object laid on a tabletop moved continuously in its longitudinal direction, in order to obtain an image whose field of view is wider than a system's own fixed imaging range. For instance, the “imaging range” is composed of a slice of several centimeters in thickness, and the “wider imaging range” than the fixed imaging range is composed of a region of some 40 centimeters assigned to the abdomen of an object.
A magnetic resonance imaging system based on a fundamental technique directed to such imaging is equipped with a radio-frequency oscillator. This oscillator is used to adjust the carrier frequency of a radio-frequency excitation pulse for selective excitation, depending on a slice-directional position of a specified section (single slice) to be imaged of an object, in cases where imaging is carried out while the object is moved.
Concretely, when a central frequency corresponding to magnetic intensity created by a magnetic resonance imaging system and a carrier frequency of an RF pulse to be applied to an object P is f0+Δf, Δf is adjusted depending on a movement of the object P. One example is to acquire an axial image while the tabletop of a patient couch is moved. To perform such imaging, the adjustable frequency Δf is given by a formula ofΔf=(γ·Gs·V·TR)/2π [Hz],in which γ is a gyromagnetic ratio, Gs is a magnitude of strength of a slice-directional gradient pulse [T/m], V is a moving speed of the tabletop [m/s], and TR is a repetition time [s].
This way of adjustment enables the position of a selectively excited slice to track a desired particular section in a continuous manner during a movement of the object. This will lead to an improved throughput, because the imaging can be done with the object moved.
The above imaging technique is also provided by another example, which is shown by the paper “A H Herlihy et al., “Continuous scanning using single shot fast spin echo on a short bore neonatal scanner,” ISMRM '98, p. 1942. This paper provides a continuous imaging technique through the control of the carrier frequency for selective excitation in connection with movement of a tabletop in performing a single shot FSE sequence.
The carrier frequency for the single shot FSE sequence is controlled on the following formula ofΔfk=Δf0+(γ·Gs·V·ETS*(k+½))/2π [Hz],in which Δf0 is an offset amount of the carrier frequency of the first excitation pulse, Δfk (≧1) is an offset amount of the carrier frequency of the k-th refocusing pulse, and ETS is an echo spacing [s].
The foregoing conventional MR imaging involving a continuous movement of an object (actually a tabletop) shows single slice imaging or sequential multi-slice imaging, which is carried out in a state where a moving direction of an object is made to agree with a direction of a slice selection axis. However, conventional MR imaging does not show the way of multi-slice imaging involving simultaneous selective-excitation of a plurality of slices. In addition, there have been no teachings about oblique imaging of multiple slices, which is one modification from multi-slice imaging, in which a slice selection axis is obliquely set to a moving direction of an object. As understood from the above, the conventional MR imaging that involves a continuous movement of an object is short of various imaging modes that will be frequently required in actual diagnosis. Thus this leads to the problem that it is difficult to perform speedy imaging, because data cannot be acquired with efficiency.
Furthermore, conventional MR imaging that involves a continuous movement of an object does not provide practical ways to reduce artifacts, which will normally be caused due to constant-speed movements of the tabletop (that is, an object).